1. Field of the Invention
The present invention relates to a gravure coating apparatus, and more particularly to a gravure coating apparatus for applying a coating liquid to a flexible substrate (hereinafter referred to as a xe2x80x9cwebxe2x80x9d) which runs continuously, the gravure coating apparatus being of a type such that no doctor blade is used.
2. Description of the Related Art
One of coating apparatuses for applying a coating liquid to a continuously running web is a gravure coating apparatus. In gravure coating, an excess amount of coating liquid supplied to a gravure roll so as to exceed a desired amount of coating liquid is usually scraped off by a doctor blade. When the doctor blade is used, however, the coating liquid forms a thin film at a coating point at which the gravure roll comes into contact with a backup roll via the web, so that a coating liquid surface in a cell of the gravure roll is liable to become of a concave shape. This presents a drawback in that the coating film applied to the web sometimes becomes irregular. To eliminate this drawback, a gravure coating apparatus without the use of a doctor blade has been contrived.
FIG. 3 shows a general construction of a conventional gravure coating apparatus without any use of a doctor blade. As shown in FIG. 3, in this gravure coating apparatus, a gravure roll 3 whose lower part is immersed in a coating liquid 2 in a liquid pan 1 is rotated to take up the coating liquid 2 from the liquid pan 1, by which the coating liquid 2 is supplied to the gravure roll 3. On the other hand, a continuously running web 4 is pressed on the gravure roll 3 by using a backup roll 6 while forming a bead portion 5 between the gravure roll 3 and the web 4, by which a desired amount of coating liquid 2 is applied to the web 4.
In the case where the gravure coating apparatus without any use of a doctor blade as described above, steady formation of bead is important for forming a satisfactory coating surface shape. For this purpose, it is necessary to excessively supply the coating liquid more than the desired coating amount to the bead portion 5. The excess coating liquid 2 that is not used for coating in the bead portion 5 flows in the direction reverse to a direction of rotation of the gravure roll 3 as a liquid return. As shown in a schematic view of FIG. 4, this liquid return forms a streak-shaped flow depending on the physical properties of the coating liquid 2 and coating speed, which poses a problem in that a streak trouble is caused on the coating surface on the web 4 due to the liquid return. Specifically, if a streak-shaped liquid return occurs, a flow in the widthwise direction is created in the bead portion 5, so that variation of size of the bead portion 5 is formed. As a result, when the web 4 is pressed on the gravure roll 3 by the backup roll 6, a force applied to the coating liquid 2 becomes nonuniform, so that unevenness of film thickness is caused on the web 4, causing the streak trouble.
As countermeasures against the streak trouble caused by the streak-shaped liquid return, Japanese Patent Application Publication No. 63-194766 discloses a method in which the coating liquid taken up from a liquid pan is brought up to the coating portion along a coating liquid guide plate, and an excess coating liquid is discharged through an excess liquid discharge guide surface, by which the liquid return flowing the roll surface is prevented.
However, in the countermeasure method disclosed in Japanese Patent Application Publication No. 63-194766, although no liquid return occurs on the roll surface of gravure roll, the coating liquid flowing along the excess liquid discharge guide surface becomes of a streak shape, which exerts an influence on the bead portion as well, so that the streak trouble is caused.
Therefore, in the conventional gravure coating apparatus of a type such that no doctor blade is used, it is an actual situation that any streak trouble cannot be solved radically.
The present invention has been achieved in view of the above situation, and accordingly an object thereof is to provide a gravure coating apparatus in which a streak trouble can be prevented from being caused on a coating surface on a flexible substrate because a liquid return of excess coating liquid that is not used for coating at a bead portion is less liable to occur.
To attain the above-described object, the present invention is directed to a gravure coating apparatus in which while a bead portion of a coating liquid is formed between a continuously running flexible substrate and a roll surface of a gravure roll to which the coating liquid more than a desired coating amount is excessively supplied, the flexible substrate is pressed on the gravure roll by using a backup roll, whereby the desired amount of coating liquid is applied to the flexible substrate, wherein a liquid supply device is provided at a position near the bead portion on an upstream side in a direction of rotation of the gravure roll to supply the coating liquid from the liquid supply device to the gravure roll.
The inventor paid attention to the fact that a force by which an excess coating liquid that is not used for coating in the bead portion is pulled in the direction reverse to the direction of rotation of roll under gravity is strong, and as the distance increases, a streak-shaped liquid return is liable to occur, and obtained a knowledge that the coating liquid is not supplied to the gravure roll by scraping the coating liquid from a liquid pan as before, but the coating liquid is supplied to a position near the bead portion on the upstream side in the direction of rotation of the gravure roll, by which the liquid return can be inhibited, so that even if the liquid return occurs, a streak-shaped liquid return can be prevented from occurring.
According to the present invention, the liquid supply device is provided at a position near the bead portion on the upstream side in the direction of rotation of the gravure roll to supply the coating liquid from the liquid supply device to the gravure roll. Therefore, even if a liquid return of excess coating liquid that is not used for coating in the bead portion is going to occur, the liquid return is blocked by the liquid supply device at a position near the bead portion, so that the liquid return in the direction reverse to the direction of rotation of the roll is less liable to occur. Therefore, the force by which the excess coating liquid is pulled in the direction reverse to the direction of rotation of roll under gravity can be weakened. Further, by supplying the coating liquid to a position near the bead portion, the coating liquid is made in a state of riding on the upper face of the roll surface and is less liable to flow down, which makes the liquid return more difficult to occur. Thereby, the occurrence of streak-shaped return liquid can be prevented effectively, so that a streak trouble can be prevented from being caused on the coating surface formed on the flexible substrate.
Also, as a preferred mode of the present invention, it is preferable that the liquid supply device has an inclined surface that is inclined downward toward the roll surface of the gravure roll to supply the coating liquid by causing the coating liquid to flow down on the inclined surface, and also has a gap larger than 0 xcexcm and not larger than 400 xcexcm between the tip end of the inclined surface and the roll surface. By causing the coating liquid to flow down on the inclined surface, the coating liquid can be supplied stably to the roll surface, and by forming the gap larger than 0 xcexcm and not larger than 400 xcexcm between the tip end of the inclined surface and the roll surface, the tip end of the inclined surface does not come in contact with the roll surface, and also the above-described effect of blocking the coating liquid can be achieved. In this case, a gap larger than 0 xcexcm and not larger than 200 xcexcm is more desirable. By making the upper limit of gap not larger than 200 xcexcm, the coating liquid can surely be prevented from flowing down through the gap, so that the liquid return of coating liquid can surely be prevented.
Thus, in the present invention, it is important that even if the liquid return of excess coating liquid in the bead portion is going to occur, the liquid return be blocked immediately to prevent the coating liquid from flowing down to the lower part of gravure roll. Therefore, it is preferable that the tip end on the roll side of the inclined surface would be located at a position such that a central angle formed between a line drawn from the center of the gravure roll to a contact point at which the flexible substrate comes into contact with the gravure roll and a line drawn from the center of the gravure roll toward the tip end on the roll surface side of the inclined surface is not larger than 45xc2x0. Also, if the angle of the inclined surface of the liquid supply device is too large, the coating liquid flowing on the inclined surface is disordered, so that it is preferable that the angle of downward inclination of the inclined surface do not exceed 15xc2x0 with respect to the horizontal.